American Options An Undergraduate Introduction to Financial Mathematics J. Robert Buchanan 2010
Early Exercise Since American style options give the holder the same rights as European style options plus the possibility of early exercise we know that C e C a and P e P a.
Early Exercise An American option (a call, for instance) may have a positive payoff even when the corresponding European call has zero payoff. S t K T t
Trade-offs of Early Exercise Consider an American-style call option on a dividend-paying stock. If the option is exercised early you, own the stock and are entitled to receive any dividends paid after exercise,
Trade-offs of Early Exercise Consider an American-style call option on a dividend-paying stock. If the option is exercised early you, own the stock and are entitled to receive any dividends paid after exercise, pay the strike price K early foregoing the interest ( ) K e r(t t) 1 you could have earned on it, and
Trade-offs of Early Exercise Consider an American-style call option on a dividend-paying stock. If the option is exercised early you, own the stock and are entitled to receive any dividends paid after exercise, pay the strike price K early foregoing the interest ( ) K e r(t t) 1 you could have earned on it, and lose the insurance provided by the call in case S(T) < K.
Parity Recall: European options obey the Put-Call Parity Formula: P e + S = C e + Ke rt
Parity Recall: European options obey the Put-Call Parity Formula: P e + S = C e + Ke rt American options do not satisfy a parity formula, but some inequalities must be satisfied.
Parity Recall: European options obey the Put-Call Parity Formula: P e + S = C e + Ke rt American options do not satisfy a parity formula, but some inequalities must be satisfied. Theorem Suppose the current value of a security is S, the risk-free interest rate is r, and C a and P a are the values of an American call and put respectively on the security with strike price K and expiry T > 0. Then C a + K S + P a
Proof Assume to the contrary that C a + K < S + P a. Sell the security, sell the put, and buy the call. This produces a cash flow of S + P a C a. Invest this amount at the risk-free rate, If the owner of the American put chooses to exercise it at time 0 t T, the call option can be exercised to purchase the security for K. The net balance of the investment is (S + P a C a )e rt K > Ke rt K 0. If the American put expires out of the money, exercise the call to close the short position in the security at time T. The net balance of the investment is (S + P a C a )e rt K > Ke rt K > 0. Thus the investor receives a non-negative profit in either case, violating the principle of no arbitrage.
Another Inequality Theorem Suppose the current value of a security is S, the risk-free interest rate is r, and C a and P a are the values of an American call and put respectively on the security with strike price K and expiry T > 0. Then S + P a C a + Ke rt
Proof Suppose S + P a < C a + Ke rt. Sell an American call and buy the security and the American put. Thus C a S P a is borrowed at t = 0. If the owner of the call decides to exercise it at any time 0 t T, sell the security for the strike price K by exercising the put. The amount of loan to be repaid is (C a S P a )e rt and (C a S P a )e rt + K = (C a + Ke rt S P a )e rt (C a + Ke rt S P a )e rt since r > 0. By assumption S + P a < C a + Ke rt, so the last expression above is positive.
Combination of Inequalities Combining the results of the last two theorems we have the following inequality. S K C a P a S Ke rt
Example Example The price is a security is currently $36, the risk-free interest rate is 5.5% compounded continuously, and the strike price of a six-month American call option worth $2.03 is $37. The range of no arbitrage values of a six-month American put on the same security with the same strike price is S K C a P a S Ke rt
Example Example The price is a security is currently $36, the risk-free interest rate is 5.5% compounded continuously, and the strike price of a six-month American call option worth $2.03 is $37. The range of no arbitrage values of a six-month American put on the same security with the same strike price is S K C a P a S Ke rt 36 37 2.03 P a 36 37e 0.055(6/12)
Example Example The price is a security is currently $36, the risk-free interest rate is 5.5% compounded continuously, and the strike price of a six-month American call option worth $2.03 is $37. The range of no arbitrage values of a six-month American put on the same security with the same strike price is S K C a P a S Ke rt 36 37 2.03 P a 36 37e 0.055(6/12) 2.03 P a 3.03
A Surprising Equality We know C a C e, but in fact:
A Surprising Equality We know C a C e, but in fact: Theorem If C a and C e are the values of American and European call options respectively on the same underlying non-dividend-paying security with identical strike prices and expiry times, then C a = C e.
A Surprising Equality We know C a C e, but in fact: Theorem If C a and C e are the values of American and European call options respectively on the same underlying non-dividend-paying security with identical strike prices and expiry times, then C a = C e. Remark: American calls on non-dividend-paying stocks are not exercised early.
Proof Suppose that C a > C e. Sell the American call and buy a European call with the same strike price K, expiry date T, and underlying security. The net cash flow C a C e > 0 would be invested at the risk-free rate r. If the owner of the American call chooses to exercise the option at some time t T, sell short a share of the security for amount K and add the proceeds to the amount invested at the risk-free rate. At time T close out the short position in the security by exercising the European option. The amount due is (C a C e )e rt + K(e r(t t) 1) > 0. If the American option is not exercised, the European option can be allowed to expire and the amount due is (C a C e )e rt > 0.
American Puts Theorem For a non-dividend-paying stock whose current price is S and for which the American put with a strike price of K and expiry T has a value of P a, satisfies the inequality (K S) + P a < K.
Proof Suppose P a < K S. Buy the put and the stock (cost P a + S). Immediately exercise the put and sell the stock for K. Net transaction K P a S > 0 (arbitrage).
Proof Suppose P a < K S. Buy the put and the stock (cost P a + S). Immediately exercise the put and sell the stock for K. Net transaction K P a S > 0 (arbitrage). Suppose P a > K. Sell the put and invest proceeds at risk-free rate r. Amount due at time t is P a e rt. If the owner of the put chooses to exercise it, buy the stock for K and sell it for S(t). Net transaction S(t) K + P a e rt > S(t) + K(e rt 1) > 0. If the put expires unused, the profit is P a e rt > 0.
Example Remark: in contrast to American calls, American puts on non-dividend-paying stocks will sometimes be exercised early. Example Consider a 12-month American put on a non-dividend paying stock currently worth $15. If the risk-free interest rate is 3.25% per year and the strike price of the put is $470, should the option be exercised early?
Solution We were not told the price of the put, but we know P a < K = 470. If we exercise the put immediately, we gain 470 15 = 455 and invest at the risk-free rate. In one year the amount due is 455e 0.0325 = 470.03 > K > P a. Thus the option should be exercised early.
American Calls Theorem For a non-dividend-paying stock whose current price is S and for which the American call with a strike price of K and expiry T has a value of C a, satisfies the inequality (S Ke rt ) + C a < S.
Variables Determining Values of American Options (1 of 3) Theorem Suppose T 1 < T 2 and then let C a (T i ) be the value of an American call with expiry T i, and let P a (T i ) be the value of an American put with expiry T i, C a (T 1 ) C a (T 2 ) P a (T 1 ) P a (T 2 ).
Proof Suppose C a (T 1 ) > C a (T 2 ). Buy the option C a (T 2 ) and sell the option C a (T 1 ). Initial transaction, C a (T 1 ) > C a (T 2 ) > 0 If the owner of C a (T 1 ) chooses to exercise the option, we can exercise the option C a (T 2 ). Transaction cost, (S(t) K) (S(t) K) = 0. Since we keep the initial transaction profit, arbitrage is present.
Variables Determining Values of American Options (2 of 3) Theorem Suppose K 1 < K 2 and then let C a (K i ) be the value of an American call with strike price K i, and let P a (K i ) be the value of an American put with strike price K i, C a (K 2 ) C a (K 1 ) P a (K 1 ) P a (K 2 ) C a (K 1 ) C a (K 2 ) K 2 K 1 P a (K 2 ) P a (K 1 ) K 2 K 1.
Variables Determining Values of American Options (3 of 3) Theorem Suppose S 1 < S 2 and then let C a (S i ) be the value of an American call written on a stock whose value is S i, and let P a (S i ) be the value of an American put written on a stock whose value is S i, C a (S 1 ) C a (S 2 ) P a (S 2 ) P a (S 1 ) C a (S 2 ) C a (S 1 ) S 2 S 1 P a (S 1 ) P a (S 2 ) S 2 S 1.
Proof (1 of 3) Suppose C a (S 1 ) > C a (S 2 ).
Proof (1 of 3) Suppose C a (S 1 ) > C a (S 2 ). Ordinarily we would argue to purchase the call C a (S 2 ) and sell the call C a (S 1 ); however,
Proof (1 of 3) Suppose C a (S 1 ) > C a (S 2 ). Ordinarily we would argue to purchase the call C a (S 2 ) and sell the call C a (S 1 ); however, the stock has only one price for all buyers and sellers, initially S(0).
Proof (1 of 3) Suppose C a (S 1 ) > C a (S 2 ). Ordinarily we would argue to purchase the call C a (S 2 ) and sell the call C a (S 1 ); however, the stock has only one price for all buyers and sellers, initially S(0). Define x 1 = S 1 S(0) and x 2 = S 2 S(0).
Proof (2 of 3) Sell x 1 options C a (S(0)) where x 1 C a (S(0)) = C a (S 1 ) and buy x 2 options C a (S(0)) where Initial transaction is x 2 C a (S(0)) = C a (S 2 ). C a (S 1 ) C a (S 2 ) > 0. If the owner of option C a (S 1 ) chooses to exercise it, option C a (S 2 ) is exercised as well. Transaction profit is x 2 (S(t) K) x 1 (S(t) K) = (x 2 x 1 )(S(t) K) > 0. Arbitrage is present.
Proof (3 of 3) Suppose P a (S 1 ) P a (S 2 ) > S 2 S 1, this is equivalent to the inequality P a (S 1 ) + S 1 > P a (S 2 ) + S 2. Buy x 2 put options P a (S(0)), sell x 1 put options P a (S(0)), and buy x 2 x 1 shares of stock. Initial transaction, x 1 P a (S(0)) x 2 P a (S(0)) (x 2 x 1 )S(0) = P a (S 1 ) P a (S 2 ) (S 2 S 1 ) > 0. If the owner of put P a (S 1 ) chooses to exercise the option, we exercise put P a (S 2 ) and sell our x 2 x 1 shares of stock. (x 2 x 1 )S(t)+x 2 (K S(t)) x 1 (K S(t)) = (x 2 x 1 )K > 0 Arbitrage is present.
Binomial Pricing of American Puts Assumptions: Strike price of the American put is K, Expiry date of the American put is T > 0, Price of the security at time t with 0 t T is S(t), Continuously compounded risk-free interest rate is r, and Price of the security follows a geometric Brownian motion with variance σ 2.
Definitions u: factor by which the stock price may increase during a time step. u = e σ t > 1 d: factor by which the stock price may decrease during a time step. 0 < d = e σ t < 1 p: probability of an increase in stock price during a time step. 0 < p = 1 2 ( ( r 1 + σ σ ) t < 1 2)
Illustration S 0 p 1 p S T u S 0 S T d S 0
Intrinsic Value Observation: an American put is always worth at least as much as the payoff generated by immediate exercise. Definition The intrinsic value at time t of an American put is the quantity (K S(t)) +.
Intrinsic Value Observation: an American put is always worth at least as much as the payoff generated by immediate exercise. Definition The intrinsic value at time t of an American put is the quantity (K S(t)) +. The value of an American put is the greater of its intrinsic value and the present value of its expected intrinsic value at the next time step.
One-Step Illustration (1 of 2) K u S 0 K S 0 K d S 0
One-Step Illustration (2 of 2) At expiry the American put is worth { P a (K us(0)) (T) = + with probability p, (K ds(0)) + with probability 1 p.
One-Step Illustration (2 of 2) At expiry the American put is worth { P a (K us(0)) (T) = + with probability p, (K ds(0)) + with probability 1 p. At t = 0 the American put is worth P a (0) = max { (K S(0)) +, e rt [ p(k us(0)) + + (1 p)(k ds(0)) +]} = max {(K S(0)) +, e rt E [ (K S(T)) +]} { } = max (K S(0)) +, e rt E [P a (T)].
Two-Step Ilustration (1 of 2) K u 2 S 0 K u S 0 K S 0 K u d S 0 K d S 0 K d 2 S 0
Two-Step Ilustration (2 of 2) At t = T/2, if the put has not been exercised already, an investor will exercise it, if the option is worth more than the present value of the expected value at t = T. { } P a (T/2) = max (K S(T/2)) +, e rt/2 E [P a (T)]
Two-Step Ilustration (2 of 2) At t = T/2, if the put has not been exercised already, an investor will exercise it, if the option is worth more than the present value of the expected value at t = T. { } P a (T/2) = max (K S(T/2)) +, e rt/2 E [P a (T)] Using the same logic, the value of the put at t = 0 is the larger of the intrisic value at t = 0 and the present value of the expected value at t = T/2. { } P a (0) = max (K S(0)) +, e rt/2 E [P a (T/2)]
Example Example Suppose the current price of a security is $32, the risk-free interest rate is 10% compounded continuously, and the volatility of Brownian motion for the security is 20%. Find the price of a two-month American put with a strike price of $34 on the security.
Example Example Suppose the current price of a security is $32, the risk-free interest rate is 10% compounded continuously, and the volatility of Brownian motion for the security is 20%. Find the price of a two-month American put with a strike price of $34 on the security. We will set t = 1/12, then u 1.0594 d 0.9439 p 0.5574.
Stock Price Lattice 35.9168 33.9019 32. 32. 30.2048 28.5103
Intrinsic Value Lattice 0 0.0981044 2. 2. 3.7952 5.48969
Pricing the Put at t = 1/12 If S(1/12) = 33.9019 then P a (1/12) = max { (34 33.9019) +, e 0.10/12 (0.5574(34 35.9168) + + (1 0.5574)(34 32) + ) } = 0.8779. If S(1/12) = 30.2048 then P a (1/12) = max { (34 30.2048) +, e 0.10/12 (0.5574(34 32) + + (1 0.5574)(34 28.5103) + ) } = 3.7942.
Pricing the Put at t = 0 P a (0) = max { (34 32) +, } e 0.10/12 [(0.5574)(0.8779) + (0.4426)(3.7952)] = 2.1513.
American Put Lattice 0 0.877945 2.15125 2. 3.7952 5.48969
Summary: P a (t) (K S(t)) + S(t) 0 0 35.9168 0.877945 0.0981044 33.9019 2.15125 2. 32. 2. 2. 32. 3.7952 3.7952 30.2048 5.48969 5.48969 28.5103
General Pricing Framework Using a recursive procedure, the value of an American option, for example a put, is given by P a (T) = (K S(T)) + { } P a ((n 1) t) = max (K S((n 1) t)) +, e r t E [P a (T)] P a ((n 2) t) = max { (K S((n 2) t)) +, } e r t E [P a ((n 1) T)] P a (0) = max. { } (K S(0)) +, e r t E [P a ( T)].
Early Exercise for American Calls If a stock pays a dividend during the life of an American call option, it may be advantageous to exercise the call early so as to collect the dividend. Example Suppose a stock is currently worth $150 and has a volatility of 25% per year. The stock will pay a dividend of $15 in two months. The risk-free interest rate is 3.25%. Find the prices of two-month European and American call options on the stock with strikes prices of $150.
Solution (1 of 3) If t = 1/12, then p = 1 2 u = e σ t d = e σ t ( ( r 1 + σ σ ) t 2) 1.07484 0.930374 0.500722.
Solution (2 of 3) Stock Prices 158.291 161.226 150 135. 139.556 114.839
Solution (3 of 3) Call Prices 8.2911 8.2911 4.00998 11.2255 1.93942 5.60565 0 0 0 0 0 0